For example, a pressurized water reactor (PWR) and a boiling water reactor (BWR) use light water as a reactor coolant (hereinafter, referred to as a “coolant”) and a neutron moderator. At the time of a normal operation of a reactor, the coolant lowers the clad temperature by cooling the cladding tube of the fuel rod.
In a heating surface between the cladding tube and the coolant, a non-boiling region is small in heat flux and sufficiently cooled. As the heat flux increases, a nuclear boiling region is increased in which vapor generated in the heating surface between the cladding tube and the coolant is converted into air bubbles and flows. In the nuclear boiling region, heat can be more efficiently transferred from the cladding tube to the coolant due to generation and agitation actions of air bubbles. As the heat flux further increases, the heating surface between the cladding tube and the coolant is covered with the vapor, heat is hardly transferred from the cladding tube to the coolant. A region in which the heating surface is covered with the vapor and so heat is hardly transferred from the cladding tube to the coolant as described above is referred to as “film boiling.” When transition from nuclear boiling to film boiling is made, the clad temperature steeply increases. Here, heat flux with which the heat transfer from the cladding tube to the coolant is lowered and the clad temperature starts to steeply increase is referred to as “critical heat flux.”
In a core thermal-hydraulic design, in order to prevent the clad temperature from steeply increasing, it is important to predict the critical heat flux. In the core thermal-hydraulic design, it is also important to compare the critical heat flux with actual heat flux inside the core and evaluate a thermal margin of a fuel rod. In the prediction of the critical heat flux, an experimental correlation based on experimental data obtained by simulating a fuel rod is used as a critical heat flux correlation.
Patent Literature 1 and Non-Patent Literature 1 disclose a critical heat flux correlation according to a related art. In the critical heat flux correlation according to the related art, for example, a linear function is used as a function of critical heat flux on a thermal equilibrium quality as stated in Paragraph [0016] of Patent Literature 1. Non-Patent Literature 2 discloses a look-up table (LUT) generated by organizing critical heat flux (CHF) of single-tube experimental data. FIG. 18 is an explanatory diagram obtained by deriving and plotting a “relation of an experimental characteristic (a look-up table of Groeneveld) of critical heat flux on a thermal equilibrium quality” based on Non-Patent Literature 2.